Image-capture system and method with two operating modes

ABSTRACT

A system for capturing at least one image of a scene. The system estimates at least one value representative of the sharpness in at least one area of at least one initial captured image of a scene. The autofocus module captures the initial image with a predefined focus. On the basis of at least the value representative of the estimated sharpness, the system selects a first operating mode or a second operating mode. In the first mode, the autofocus module controls the focus of the system to capture a sharper image of the scene. In the second mode, the initial captured image or another captured image is processed by the digital processing unit, the other image being captured with the predefined focus.

RELATED APPLICATIONS

This application is a §371 application from PCT/FR2010/051671 filed Aug.6, 2010, which claims priority from French Patent Application No. 0955593 filed Aug. 10, 2009, each of which is herein incorporated byreference in its entirety.

TECHNICAL FIELD OF THE INVENTION

This invention relates to the field of digital image-capture systems(fixed or animated) such as those contained in picture-taking devices,cameras, camera phones (mobile telephones capable of taking photos),scanners, faxes, endoscopes, surveillance cameras, toys, personalassistants, computers, heat cameras, ultrasound apparatuses, MRI(magnetic resonance imaging) apparatuses and X-ray apparatuses, etc.

It classically comprises a sensor with a plurality of light-sensitiveelements, known as pixels, which transform a quantity of light receivedinto digital values, and an optical system focusing the light towardsthe sensor.

The sensor can, for example, be a CCD (Charged Coupled Device), a CMOS(Complementary Metal Oxide Semiconductor), a CID (Charge InducedDevice), an IRCCD (Infra-Red CCD), an ICCD (Intensified CCD), an EBCCD(Electron Bombarded CCD), an MIS (Metal Insulator Semiconductor), an APS(Active Pixel Sensor), a QWIP (Quantum Well Infrared Photo detector), anMQW (Multiple Quantum Well), a sensor sensitive to the light spectrumand/or to other electromagnetic spectral bands or other device. It canbe associated with a Bayer filter in order to obtain a color image.Given that some of these sensors are capable of counting several millionpixels, the unit currently used to count the pixels of a sensor is theMegapixel.

More precisely, the invention relates to the problem of obtaining sharpimages by these image capture systems.

BACKGROUND OF RELATED ART

Image capture systems are known to comprise an optical system, a digitalimage sensor and an autofocus module.

Optical system refers to an optical system comprising one or severalelements in particular including lenses, mirrors and/or diffractiveelements.

The autofocus module comprises an automatic adjustment function foradjusting the system's focus so as to obtain a sharp image.

The focusing operation consists in converging the light rays exiting theoptical system at the image sensor. The term converge involvesminimizing the size of the point spread function obtained at the imagesensor.

This focus is generally determined by successive iterations, by browsingthrough the different configurations accessible with the autofocusmodule and by measuring the sharpness of each of these configurations,for example for the green color component when this is a color image.

It should be noted that in this document, the terms “color component”and “color” correspond to the same notion.

The use of the autofocus module however presents two main disadvantages:the time taken to determine the optimal configuration can be long (thistime is referred to as autofocus latency) and the amount of electricityconsumed in particular to produce movements is high.

FIG. 5 demonstrates the electrical consumption of an image capturesystem with an autofocus module from the prior art: the curve L1 betweenthe points J1 and K1 represents the electrical intensity valuesaccording to the defocus values in the case of a system focusing betweenan object placed at a focus distance of infinity and a close object. Thepoint J1 corresponds to zero defocus (object at a distance of infinity)and the point K1 corresponds to maximum defocus (close object).

This figure is provided for illustration purposes with the level ofelectrical consumption required for defocus capable of varying accordingto the type of autofocus module used.

Moreover, this is why the autofocus module is not activated in mostcamera phones (it is set for an object at a distance of infinity) whenthe user uses the camera phone in preview mode (i.e. when the image isonly captured at a low resolution (the resolution refers to the numberof pixels contained in the image): for example 320 pixels*400 pixels atmost, before the true image capture taken by the user, at a higherresolution). The autofocus mode is only activated at the time when thepicture is taken by the user. The latter therefore a priori does notknow whether the image taken at this instance will be sharp or blurred.

SUMMARY AND OBJECT OF THE INVENTION

A solution is therefore required for obtaining sharp images whileminimizing energy consumption and autofocus latency.

To this end and according to a first aspect, the invention proposes asystem for capturing digital images intended to capture at least oneimage of a scene, said system including an optical system, a sensor, anautofocus module for adjusting the focus of the system and digitalprocessing means.

This system is designed to estimate a value representative of thesharpness in at least one area of at least one initial captured image ofa scene, the autofocus module being positioned so as to capture theinitial image with a predefined focus. On the basis of at least thevalue representative of the estimated sharpness, the system can selectan operating mode from between a first mode and a second mode. In thefirst mode, the autofocus module is used to control the focus of thesystem in order to capture a sharper image of the scene; and in thesecond mode, the initial captured image or another captured image isprocessed by the digital processing means, the other image beingcaptured with said predefined focus.

Such a system improves the sharpness of the images provided by the imagecapture system while reducing the latency and energy consumptionrequired, by using digital processing operations where appropriate andby reserving the use of the autofocus module for cases where it iscompulsory to improve sharpness.

The focus adjustment is generally performed using an actuator (voicecoil technology, stepper motor, piezoelectric device, MEMS(Microelectromechanical Systems), USM (Ultra Sonic Motor) or any othermeans) to move the optical system and/or the sensor, or even to moveelements within the optical system, or by using liquid lenses or anyother electrical, mechanical and/or magnetic means modifying the shapesand/or positions of the elements making up the optical system, so thatthe sensor is in the plane of focus of the image of the scene captured.

The use of a liquid crystal phase modulator device (PDLC) is anotherexample of modifying the properties of the optical system to adjustfocus.

The invention includes the step of positioning the autofocus module tocapture an image with a predefined focus. Predefined focus can beunderstood as the focus obtained when the autofocus module is notactivated and advantageously any position in which focus remains stablewithout consuming electricity or by consuming as little as possible.Predefined focus can be obtained when the position of the autofocusmodule is at rest or in a position obtained after adjusting the focus.

Predefined focus can advantageously involve hyperfocal focusing forobtaining sharp images at a distance of infinity with a maximum depth offield, or another focus determined for example in order to maximize thedepth of field of the second mode. Predefined focus can also depend onan adjustment made by the user or an automated adjustment, for exampleopening or changing the focal distance of the optical system. Predefinedfocus can also describe any position obtained after turning on thesystem or obtained during system operation and in particular afterimplementing the first mode.

The invention implements image processing means. The image processingmeans can be integrated into the sensor and the optical system orlocated in a separate apparatus connected to the image capture system bytransmission means.

Image processing means refer to software receiving one or several imagesand performing processing operations on these images.

These processing operations can in particular include:

-   -   calculating a corrected image, and/or    -   performing measurements, and/or    -   combining several images, and/or    -   improving the accuracy of images, and/or    -   improving the subjective quality of images, and/or    -   detecting objects or persons in a scene, and/or    -   adding objects or persons to a scene, and/or    -   replacing or modifying objects or persons in a scene, and/or    -   removing shadows in a scene, and/or    -   adding shadows to a scene, and/or    -   researching objects in an image base.

In one embodiment, the other image is for example captured subsequentlyto the initial image capture.

In one embodiment, the system is intended to capture a sequence ofimages and also comprises a device which, when at least one subsequentimage is captured with the predefined focus or another focus and on thebasis of at least one value representative of the sharpness estimatedfrom the subsequent image, can select an operating mode from between thefirst mode and the second mode, in the second mode, the subsequent imageor another image captured being processed by the digital processingmeans, the other image being captured with the same focus as thesubsequent image.

In one embodiment, in the second operating mode, the digital processingmeans are capable of applying a digital image processing operationintended to improve the sharpness of the initial image, the subsequentimage or another image captured, without implementing the autofocusmodule. In such an embodiment, the first and second operating modes aredesigned to obtain a sharper image of the scene.

In one embodiment, this other image is for example captured subsequentlyto the initial image capture.

In one embodiment of the image capture system according to theinvention, the value representative of the sharpness is determinedaccording to:

-   -   a relation between the levels of sharpness of at least two        respective color components in the initial image; and/or    -   the comparison between the respective levels of sharpness of at        least two color components in the initial image.

These provisions enable the operating mode to be selected according tothe measurements performed in the initial image only.

In one embodiment, the image capture system is also capable, afterselecting the first operating mode, of choosing a focus adjustmentaccording to:

-   -   a relation between the levels of sharpness of at least two        respective colors determined according to the initial image;        and/or    -   an astigmatism measurement in at least one area of the initial        image and on at least one color.

This provision therefore reduces the number of required iterationsimplemented by the autofocus function, thus reducing the latency andenergy consumption required.

In one embodiment, the image capture system is also capable, afterselecting the second operating mode and in at least one region of theimage processed by the digital processing means, of identifying asharper color component and a more blurred color component and ofmodifying the pixel intensity of the image processed for the moreblurred color according to the pixel intensity of the image for thesharper color.

This provision therefore easily improves the sharpness of the imageprocessed via digital processing.

According to a second aspect, the invention relates to a method forcapturing images intended for a system including an optical system, asensor, an autofocus module for adjusting the focus of the system anddigital processing means, containing the following steps:

-   -   estimating a value representative of the sharpness in at least        one area of at least one initial captured image of a scene, said        autofocus module being positioned so as to capture the initial        image with a predefined focus; and on the basis of at least the        value representative of the estimated sharpness, selecting an        operating mode from between a first mode and a second mode,    -   whereby in the first mode, the autofocus module is used to        control the focus of the system in order to capture a sharper        image of the scene; and    -   in the second mode, the initial captured image is processed by        the digital processing means, with the other image being        captured with said predefined focus.

According to a third aspect, the invention relates to a computer programand/or electronic component to be installed within a processing unit ofan image capture system intended to capture at least one image of ascene and including an optical system, a sensor, an autofocus module foradjusting the focus of the system, said program comprising instructionsand/or said component comprising circuits to implement the steps of amethod according to the second aspect of the invention upon theexecution of the program and/or activation of the component by saidprocessing module.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will be discoveredafter reading the following description. This is purely forillustrational purposes and must be read using the appended figures, inwhich:

FIG. 1 represents an image capture system 1 according to one embodimentof the invention;

FIG. 2 represents the steps of a method according to the invention;

FIG. 3 illustrates the variations in sharpness of the image of an objectaccording to different color components and according to the distanceseparating the object and the camera;

FIG. 4 represents the relative sharpness between two color components ofan image according to image defocus;

FIG. 5 illustrates the electrical consumption of an image capture systemfrom the prior art, with a standard autofocus module according todefocus;

FIG. 6 illustrates the electrical consumption of an image capture systemof the invention according to defocus.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 represents an image capture system 1 according to one embodimentof the invention, for example a camera.

The camera 1 comprises an optical system 2, a sensor 3, an autofocusmodule 4 and a processing unit 5.

The autofocus module 4 is capable of automatically adjusting focusaccording to an image of the scene, in this case for example by varyingthe distance between the optical system 2 and the sensor 3 using a motor(other types of focus such as those previously mentioned can however beimplemented in the invention).

The sensor 3 is equipped with sensitive elements for converting a flowof photons into an electric signal. In the embodiment considered, thesensitive elements are sensitive to three colors. The digital imagecaptured is therefore a color image containing three respective colorcomponents.

It should be noted that the invention can be implemented with two ormore colors.

In this embodiment, a captured image is considered to contain one bluecolor component C1 (wavelengths of substantially between 400 and 500nm), one green color component C2 (wavelengths of substantially between500 and 600 nm) and one red color component C3 (wavelengths ofsubstantially between 600 and 700 nm).

In other embodiments, other components (for example infrared,ultraviolet, etc.) and/or any number of components more than or equal totwo can be considered when implementing the invention.

Two main points must be recalled.

Firstly, the focus point of the optical system 2 is specific to eachcolor, so that a focus point O1 specific to one color C1 is differentfrom the focus point O2 of another color C2. Consequently, the imageformed by a second color at the sensor is sharper or more blurred thanthe image formed by a first color, which reduces the sharpness of theoverall image formed by the sensor.

Secondly, the focus point of the optical system 2 for a given wavelengthvaries according to the distance at which the scene represented in theimage is located.

FIG. 3 illustrates the variations in sharpness of the color componentsC1, C2 and C3 of an object captured according to the distance d betweenthe optical system 2 and the object being captured. The color componentsC1, C2 and C3 correspond for example to the blue, green and redcomponents respectively. The abscissa represents the distance d and theordinate represents the diameter T of the point spread function.

The range of distances d has been subdivided into three parts: the partwhere the blue component is the sharpest is the part PI, referred to asthe “macro” part (typically less than 60 cm), the part PII where thegreen color component is sharper than the blue component and the redcomponent is referred to as the “portrait” part (typically between 60 cmand 4 m) and the part PIII where the red component is sharper than theblue component and the green component is referred to as the “landscape”part (typically more than 4 m).

It should be noted that this specific order of focus between the colorsis only provided for illustration purposes. The invention can beimplemented for any order of focus between the colors, in particular insystems with chromatic inversion (produced for example using adiffractive object).

When considering an initial color image I of a scene captured by thecamera 1, with the autofocus module 4 being set for an object at adistance of infinity or beyond (indeed, autofocus modules are oftenadjusted to distances beyond infinity in order to guarantee sharpness ata distance of infinity despite thermal differences and positioningerrors which cause defocus).

Camera 1 is designed to implement the following steps, described withreference to FIG. 2.

In a step 101, the processing unit 5 performs at least one sharpnessmeasurement on a determined area Z in the image I, with the autofocusmodule 4 being positioned so as to capture the initial image with apredefined focus.

In one embodiment, for this measurement, the optical system isoriginally set to hyperfocus in order to benefit from a larger depth offield in the second mode (M2).

In one embodiment, the area Z is comprised of the entire image I and inother embodiments, the area Z corresponds to a selected group of pixels.

According to the sharpness measured and a selection rule R defined inparticular according to the sharpness measured, the processing unit 5selects an operating mode from between the mode M1 and the mode M2.

If the operating mode selected in step 101 is the mode M1, in step 102,the autofocus module 4 is activated so as to obtain an image of thescene with improved sharpness. An iterative focus adjustment process,for example an optimal distance between the optical system 2 and thesensor 3, corresponding to optimal sharpness (or at least to a level ofsharpness greater than a determined minimum threshold) is thereforeimplemented, by calculating, for each modification in the distanceseparating the optical system 2 from the sensor 3, a respectivemeasurement representative of the sharpness of the image of the scenecaptured for the considered distance separating the optical system 2from the sensor 3.

If the operating mode selected in step 101 is the mode M2, in step 103,digital processing operations are applied by the processing unit 5 onthe initial image of the scene captured (or potentially on anotherimage), for example so as to obtain an image of the scene with improvedsharpness, without implementing the autofocus function.

The content of the different steps is provided in more detail below.

Step 101

The sharpness measurement in the area Z of the image I performed by theprocessing unit 5 in step 101 can be an overall measurement on the imageor on the other hand, vary according to color components considered onan individual basis.

In one embodiment of step 101, an overall sharpness measurement isperformed and the selection rule R corresponds, for example, to thecomparison between the sharpness measured and a determined threshold. Ifthe sharpness measured is beneath the determined threshold, theoperating mode M1 is selected. Otherwise, the operating mode M2 isselected.

In another embodiment of step 101, the processing unit 5 performs asharpness measurement for a color component and selects an operatingmode from between modes M1 and M2 by comparing the sharpness measurementto a determined threshold.

In another embodiment of step 101, the processing unit 5 performs asharpness measurement for several color components and selects anoperating mode from between modes M1 and M2 by comparing the sharpnessmeasurements to the respective determined thresholds.

In another embodiment of step 101, the processing unit 5 performs asharpness measurement according to each of two different directions andselects an operating mode from between modes M1 and M2 by comparing thesharpness measurements.

In another embodiment of step 101, the processing unit 5 performs arelative sharpness measurement between at least two color components.For example, it determines which color component is the sharpest inimage I between the blue component and the green component. Then, if theblue component is the sharpest of the two blue and green components, theprocessing unit 5 selects the operating mode M1. Otherwise the operatingmode M2 is selected.

This aforementioned embodiment for step 101 resorts to using theautofocus module 4 for a captured scene located in the “macro” PI partand to using a unique digital processing operation without implementingthe autofocus module 4 for captured scenes located in the “portrait” PIIand “landscape” PIII parts, with the selection between the two modes M1and M2 being simply made on the basis of a comparison between the levelsof sharpness of two color components.

For other embodiments, the mode M1 can be selected, for example for the“macro” and “portrait” parts, and the mode M2 for the “landscape” part.

In one embodiment, the operating mode M1 is selected when the twofollowing conditions Cond1 and Cond2 are met:

Cond1: the scene is located in the macro area (i.e. the blue componentis sharper than the green component);

Cond2: the sharpness of the blue component also being beneath adetermined threshold.

Alternatively, the condition Cond2 takes into account the relativesharpness of two or more color components, and therefore variesaccording to a relationship between the levels of sharpness of severalcolor components.

In another embodiment, the operating mode selected varies, for example,according to the comparison made between a threshold and a function ofthe levels of sharpness of different color components, for example thedifference in sharpness between the components C1 and C3 divided by thesharpness of the component C2.

A relative sharpness measurement can vary according to a quotient or adifference between the sharpness measurement for a color component andthe sharpness measurement for another color component.

The relative sharpness measurement of one color component compared toanother is advantageous in relation to an absolute sharpness measurementfor the image, as the former is more precise and does not depend on thecontent of the scene observed. Indeed, an absolute measurement is notreliable enough in some cases to determine whether mode M1 or M2 shouldbe selected. For example, an absolute sharpness measurement can operateperfectly for a scene with high contrast such as a test chart, whereasit will not be reliable if the content of the scene is smooth.

Therefore, the use of an optical system is advantageous (for example alens) employing longitudinal chromatic aberration in order to perform arelative sharpness measurement of a color component compared to anotherand have a more stable measurement that does not depend on the sceneobserved.

Other selection rules can be defined, for example combining the rulesprovided hereinafter or implementing other rules.

Furthermore, the invention applies with various known sharpnessmeasurements, per color component or in an overall manner. A sharpnessmeasurement can correspond to a scalar value or a vector.

For example, the sharpness of a color can correspond to the measurementof a value referred to as BXU which is a blur experience measurement asdescribed in the article published in the “Proceedings of IEEE,International Conference of Image Processing, Singapore 2004” andentitled “Uniqueness of Blur Measure” by Jérôme BUZZI and FrédéricGUICHARD.

Sharpness can be indicated according to the variance in the point spreadfunction or even using the MTF (Modulation Transfer Function), which isthe Fourier transform of the point spread function.

Various measurement methods for measuring such sharpness are describedin manuals and publications such as, for example, the “Handbook of image& Video processing” edited by Al Bovik and published by Academic press,pages 415 to 430.

In one embodiment, the sharpness of a color component is obtained bycalculating a gradient.

For example, a sharpness value is estimated for the pixels of the area Zof the digital image by performing for each or some of the pixels P, ameasurement M in the pixel P, for a given color component C,corresponding to the variation gradient of C in a neighborhood of P.This is obtained by the following calculation:

For a given color C, V(P) is considered to represent a neighborhood ofthe pixel P.

GM is noted, representing the median magnitude of the gradients in theneighborhood V(P), and SM representing the median magnitude of thedifferences between GM and the gradients in the neighborhood V(P).

A gradient is calculated by the magnitude of the difference inintensities between two pixels of the same color. The gradients in theneighborhood V(P) correspond to the gradients involving a predeterminednumber of pixel pairs in the neighborhood V(P).

The measurement M representative of the sharpness of the pixel P havinga color C can be defined by the relationship between SM and GM. A valueM (P, C) is thus obtained.

Whereas in an image capture apparatus with a standard autofocus module,the autofocus module must be activated in order to determine whether theimage is sharp, which involves the acquisition of at least two images tocompare their level of sharpness, an image capture apparatus accordingto this invention defines, with the capture of a single image, whetherthe activation of the autofocus module is required or not.

Step 102:

In one embodiment, when the operating mode Ml is selected, theprocessing unit 5 calculates a focus adjustment instruction, for examplea distance to be applied between the optical system 2 and the sensor 3for implementing the autofocus function and provides this to theautofocus module 4. This focus adjustment instruction is, for example,calculated according to one or several sharpness measurements performedbefore using the autofocus module, for example a sharpness measurementperformed in step 101.

For example, as illustrated in FIG. 4, based on the relative sharpnessbetween two color components (in this case a graph drawn for the camera1 representing the value A of the difference between the sharpness ofthe color C1 and that of the color C3 divided by the sharpness of thecolor C2 and the opposite difference) an image defocus value can bededuced, this defocus representing the focus adjustment to beimplemented, in this case the distance variation to be applied betweenthe sensor 3 and the optical system 2).

This provision accelerates the convergence of the iterative processimplemented by the autofocus module 4 and therefore reduces the latencyand energy consumption required.

In another embodiment, the dependence of astigmatism can be used withthe focus in order to further reduce latency and more particularly whenthe value Δ is no longer linear with the defocus.

In the presence of astigmatism in an optical system, the shape of thepoint spread function depends on the focus. On either side of the focuspoint where the point spread function is symmetrical, one direction willbe preferred when compared to another, which will lead to a sharp imagein one direction and a blurred image in a perpendicular direction.Consequently, according to the direction of sharpness in an image, thiscan be connected to the defocus. For a symmetrical system withastigmatism that is not fully corrected (concerning a large majority ofoptical systems), this will appear in the field. During the design phasefor the optical system, a correlation table can be drawn up between anastigmatism measurement and/or its direction with defocus of the opticalsystem. Furthermore, for optical systems containing high levels ofmisaligned optical elements, a calibration phase can be considered,connecting an astigmatism measurement and/or its direction on theoptical axis (centre of the image) with defocus. This is possiblebecause, when a system is comprised of elements misaligned in relationto each other, astigmatism is present in the centre of the image.

Furthermore, this defocus measurement, using the astigmatism, isrelatively insensitive to the contents of the scene, as the astigmatismdiffers according to the color component considered, whichdifferentiates between astigmatism originating from the optical systemand therefore characteristic of defocus and that originating from thescene, which does not depend on the color component.

Step 103:

In one embodiment, when the operating mode M2 is selected, theprocessing unit 5 implements digital processing operations for theinitial image I and does not implement the autofocus function.

The digital processing operations performed can vary: for example shaperecognition, white balance adjustment, tone curve adjustment,dematrixing, etc.

In one embodiment, the digital processing operations correspond to theprocessing operations described in document PCT/FR2006/050197, adaptedto improve the sharpness of at least one color component by choosingfrom the image's colors at least one color component referred to as the“sharp color” and by reflecting the sharpness of the sharp colorcomponent onto at least one other more blurred color component.

Therefore, by considering the aforementioned example of embodiment,wherein the operating mode M2 was selected after determining that thesharpness of the blue color component C1 was lower than the sharpness ofthe green color component C2.

In the following step, the sharpest color component is determined frombetween the color component C2 and the color component C3. The colorcomponent determined as the sharpest is hereinafter referred to as thesharp color and the two other color components are hereinafter referredto as the components requiring improvement.

In this example, CA, CO and CN are the respective intensities of apixel, characterized by grey levels and representative of the improvedcolor component, the color component requiring improvement and the sharpcolor component respectively.

The sharpness of the sharp color component is reflected onto the colorcomponent requiring improvement using a filter F according to a formulasuch as:CA=CN+F(CO−CN)

This removes the high frequencies of the color component requiringimprovement and replaces these with the high frequencies of the sharpcolor component.

Typically, the filter F will have the specific function of removing thedetails of the image on which it is applied. In order to achieve this, alinear low-pass (or median) filter can be used. Many known non-linearfilters can also be used with the specific function of removing detailsin a similar manner to that of a median filter.

It should be reminded at this stage that the human retina has aparticularly high level of sensitivity, with regard to the details of animage, to the green color such that the adjustment of optical systemsgenerally aims at obtaining a high level of sharpness for this colorover a certain adjustment range (for example, refer to pages 30 to 33 ofthe works “Color appearance models” by Mark D. Fairchild edited byAddison Wesley).

Therefore, according to an observation specific to this invention, anoptical device producing images, the sharpness of which is notsatisfactory to the human eye, can present a satisfactory level ofsharpness for one of its colors, such as the color blue or red, forwhich the eye is less sensitive when considering the details.

Typically, for an optical system focused at a hyperfocal distance,considering an image with a close object and a faraway object, thesharpness of the faraway object is generally often determined using agreen color whereas the sharpness of the close object is improved bytaking into account the blue color.

It is therefore important that the areas of an image can be improvedaccording to different sharp colors according to the relative sharpnessbetween two colors.

Therefore, in one embodiment of the invention, the image I is brokendown into areas. Then, by successively considering each area, thesharpest color for that area is identified. In one embodiment, thesharpness of the sharpest color is reflected onto the other colorcomponents in the area considered. In another embodiment, an overallcorrection is applied to the image, which is calculated using weightfactors as a weight factor is representative of the number of times thatan associated sharp color has been identified in the areas.

Such a processing operation performed according to the characteristicsof the different areas of the image is advantageous when compared to theuse of an autofocus module within an image capture system from the priorart. Indeed, in the example of a scene with objects located at differentdistances, an image capture system from the prior art will focus on oneof these objects alone, whereas according to the invention, a sharpimage is obtained for all objects considered.

In one embodiment, the processing operations described in steps 101, 102and 103 are implemented by the processing unit 5 after the execution ofa program comprising instructions for implementing the steps incumbentupon the processing unit 5 during the execution of the program by thecalculation means of the processing unit 5.

In the aforementioned embodiment, the sharpness improvement step 103 hasbeen performed on the initial image I captured after selecting theoperating mode M2.

In another embodiment, the improvement process in step 103 is performedon one or several other images, for example images captured after theinitial image, for example in the event of video images with the otherimage being captured with the predefined focus.

The implementation of the invention produces particularly advantageousresults when capturing video images.

Indeed, the latency caused by the iterative functioning of the autofocusmodule was very disadvantageous when capturing video images, above allwhen the object captured was moving, which also produced oscillationphenomena in terms of focus when viewing adjusted images.

This invention considerably reduces the oscillation phenomenaencountered and the latency time required to obtain a sharp image whencapturing video images. Indeed, it considerably reduces the use of theautofocus module when the operating mode M2 can be selected, which isparticularly beneficial with moving objects. This results in a certainmanner to stabilizing the image.

Stabilization is increased further when a prior focus adjustmentdetermined according to a sharpness measurement performed is previouslyapplied to the autofocus module, as indicated in the aforementioneddescription for step 102.

The invention also increases the level of reliability of the imagestabilization operation, intended to compensate for the level of blurcaused by movement, not of the scene itself but of the opticalsystem/sensor, using a digital and/or optical correction process (bymoving the lenses for example).

By being much faster to converge in mode M1, the stabilizationmeasurement performed on a sharp image is more reliable. In mode M2,without using the autofocus module, the image is sharp and themeasurement used for image stabilization is more reliable.

Therefore, as the latency is reduced, the sharp image is more quicklyobtained and therefore the stabilization measurement is more reliable.With regard to autofocus modules from the prior art, the oscillationscaused by the dichotomy when moving itself into the correctconfiguration makes the image stabilization measurement difficult.

FIG. 6 demonstrates the electrical consumption of an image capturesystem according to the invention: the curve L2 between the points J1and K1 represents the electrical intensity values according to thedefocus values (Def) in the case of an optical system focusing between afocus distance of infinity and a given close distance. The point J1corresponds to zero defocus (object at a distance of infinity) and thepoint K1 corresponds to maximum defocus (close object).

The vertical dotted line indicated by the arrow marks the passage fromthe operating mode M2 (to the left of this vertical line) to theoperating mode M1 (to the right of this vertical line).

Therefore, in the operating mode M1, the electrical consumption withinan image capture system according to the invention is equal to that of asystem from the prior art for the same level of defocus (refer to FIG.5). However, in the operating mode M1, thanks to the prior supply of afocus instruction, the curve L1 is not travelled several times withinthe scope of an iterative dichotomy process in order to determine thefocus position such as in the prior art.

For all defocus values in the operating mode M2, the electricalconsumption within an image capture system according to the invention isequal to that of a system from the prior art for zero defocus, i.e. atminimum consumption.

For example, in one aforementioned embodiment where the operating modeM2 corresponds to the Portrait and Landscape parts, and the operatingmode M1 corresponds to the Macro part, such minimum consumption occursfor scenes located at a distance from the system of between a distanceof infinity and approximately 60 cm, which corresponds to most uses madeof image capture systems.

This invention therefore restricts the use of the autofocus module to alimited number of cases, when sharpness improvement is not sufficient orappropriate when performed by a digital processing operation, thusenabling the sharpness of a sharper color to be transferred to one orseveral more blurred colors. The invention therefore improves thesharpness of the images captured while limiting energy consumption andautofocus latency.

An autofocus adjustment can therefore be performed in camera phones,even in preview mode. Furthermore, when taking videos, the successiveimages captured can also be made sharp (which was not possiblebeforehand, with the adjustment operation performed by the autofocusmodule upon each video image not being possible due to the rapidsuccession of images).

The invention claimed is:
 1. A system for capturing at least one imageof a scene, comprising: an optical system; a sensor; an autofocus moduleto adjust a focus of the optical system and to capture at least oneinitial captured image of a scene with a predefined focus; and a digitalprocessing unit to estimate at least one value representative of thesharpness in at least one area of said at least one initial capturedimage of the scene; wherein the system selects a first operating mode ora second operating mode based on said at least one value representativeof estimated sharpness; wherein the autofocus module is activated tocontrol the focus of the optical system to capture an image of the scenewith improved sharpness in the first operating mode; and wherein thedigital processing unit processes said at least one initial capturedimage or another image captured with the predefined focus in the secondoperating mode.
 2. The system of claim 1, wherein the sensor captures asequence of images; wherein the digital processing unit selects betweenthe first and second operating mode when at least one subsequent imageis captured by the sensor and based on said at least one valuerepresentative of the sharpness estimated from said at least onesubsequent captured image; and wherein the digital processing unitprocesses said at least one subsequent captured image or another imagecaptured with a same focus as said at least one subsequent image in thesecond operating mode.
 3. The system of claim 1, wherein, the digitalprocessing unit applies a digital image processing operation to improvethe sharpness of said at least one initial captured image, a subsequentcaptured image or another captured image without invoking the autofocusmodule in the second operating mode.
 4. The system of claim 1, whereinthe digital processing unit determines said at least one valuerepresentative of the sharpness according to at least one of thefollowing: a relation between levels of sharpness of at least tworespective color components in said at least one initial captured image;or a comparison between respective levels of sharpness of at least tworespective color components in said at least one initial captured image.5. The system of claim 1, wherein the autofocus module adjusts the focusof the optical system in the first operating mode according to at leastone of the following: a relation between levels of sharpness of at leasttwo respective color components in said at least one initial capturedimage; or an astigmatism measurement in said at least one area of saidat least one initial captured image and on at least one color.
 6. Thesystem of claim 1, wherein the digital processing unit identifies asharp color component and a blurred color component in at least oneregion of an image processed by the digital processing unit in thesecond operating mode, and modifies pixel intensity of the blurred colorcomponent of the image according to the pixel intensity of the sharpcolor component of the image.
 7. A method for capturing images by animage capture system comprising an optical system, a sensor, anautofocus module for adjusting the focus of the optical system and adigital processing unit, comprising the steps of: estimating at leastone value representative of sharpness in at least one area of at leastone initial image of a scene captured by the autofocus module with apredefined focus by the digital processing unit; selecting a first orsecond operating mode by the digital processing unit based on said atleast one value representative of the estimated sharpness; activatingthe autofocus module to control a focus of the optical system and thesensor to capture an image of the scene with improved sharpness in thefirst operating mode; and processing said at least one initial capturedimage or another image captured with the predefined focus by the digitalprocessing unit in the second operating mode.
 8. The method of claim 7,further comprising the steps of: capturing a sequence of images by thesensor; selecting between the first and second operating mode by thedigital processing unit when at least one subsequent image is capturedby the sensor and based on said at least one value representative of thesharpness estimated from said at least one subsequent captured image;and processing said at least one subsequent captured image or anotherimage captured with a same focus as said at least one subsequentcaptured image by the digital processing unit in the second operatingmode.
 9. The method of claim 7, further comprising the step of applyinga digital image processing operation to improve the sharpness of said atleast one initial captured image, a subsequent captured image or anothercaptured image by the digital processing unit without invoking theautofocus module in the second operating mode.
 10. The method of claimof claim 7, further comprising the step of determining said at least onevalue representative of the sharpness by the digital processing unitaccording to at least one of the following: a relation between levels ofsharpness of at least two respective color components in said at leastone initial captured image; or a comparison between respective levels ofsharpness of at least two respective color components in said at leastone initial captured image.
 11. The method of claim 7, furthercomprising the step of adjusting the focus of the optical system in thefirst operating mode by the autofocus module according to at least oneof the following: a relation between levels of sharpness of at least tworespective color components in said at least one initial captured image;or an astigmatism measurement in said at least one area of said at leastone initial captured image and on at least one color.
 12. The method ofclaim 7, further comprising the step of adjusting the focus of theoptical system in the first operating mode by the autofocus moduleaccording to an astigmatism measurement in said at least one area ofsaid at least one initial captured image and on at least one color. 13.The method of claim 7, further comprising the step of identifying asharp color component and a blurred color component in at least oneregion of an image processed by the digital processing unit in thesecond operating mode; and modifying pixel intensity of the blurredcolor component of the image by the digital processing unit according tothe pixel intensity of the sharp color component of the image.
 14. Themethod of claim 7, wherein the steps are performed by a computerexecutable program installed in the digital processing unit of the imagecapture system.
 15. The method of claim 7, wherein the steps areimplemented by an electronic component within the digital processingunit.